NCARB PDD Exam - Topic 3 Question 11 Discussion

In a tropical climate, the interior is cooler and drier than the hot, humid exterior. The vapor drive is from outside inside, so the vapor retarder must be installed on the warm/moist side of the assembly, which is below the insulation when the insulation is above the roof deck. This prevents moist exterior air from reaching cooler surfaces inside the insulation where condensation could occur.

PDD Reference: Psychrometrics & vapor drive principles, PDD ''Thermal & Moisture Protection---Placement of vapor barriers,'' ASHRAE Handbook recommendations.